Buchholz trip due to a downstream phase to phase fault 2

[PolyMike]

We had recently a blackout due to a Buchholz trip of a 132/6,3kV YNyn 18,75MVA due to a downstream phase to phase short-circuit.

I knew that occasionally Buchholz trip might be faster than phase protection relays; but only in case of an internal fault. In this case, there are 3 phase protection relays between the transformer and the fault. All CEE relays : 1 ITG7392 and 2 RMS791.
Relay settings seems to be o.k. and selectives among themselves. We'll carry a test one by one, but I hardly believe all of them failed. We are expecting the DGA results of the Buchholz oil in order to rule out a damage in the transformer. This is the first Buchholz trip of the transformer for years.

Does anybody know any case of Buchholz trip in a downstream fault?

Thank you in advance...

 

[davidbeach]

Winding movement due to through fault is a likely suspect.

 

[dpc]

These things can trip when the wind shifts. To be effective, they have to be pretty sensitive. A heavy through fault can sometimes cause enough vibration and movement internally to do the trick, as davidbeach mentioned.

Probably need to run through test procedures for the Buchholz relay at the first opportunity.

But nuisance tripping of these devices and sudden pressure relays is, unfortunately, fairly common.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[prc]

When a LL fault occurs, fault current in transformer cause the windings to oscillate axially.This results in an oil surge in Buccholtz relay.The solution is to slightly desensitise the relay.Tank vibrations also can cause relay to operate esp with mercury switches.Solution -check the clamping of oil pipe on which relay is mounted.In this case,ensure that transformer has not failed by doing DGA.

 

[stevenal]

http://www.beckwithelectric.com/infoctr/techpapers/xfmr_en.pdf

See the section on sudden pressure relay blocking. Don't see why this could not also be used with your Buchholz.